This invention relates to semiconductor devices including a Schottky barrier and having an improved voltage blocking characteristic, particularly but not exclusively Schottky diodes designed for fast switching and blocking of medium or high voltages.
Schottky diodes with high breakdown voltages are described in the article of that title by B. M. Wilamowski in Solid State Electronics, Vol 26, (1983), No. 5, pages 491 to 493. The disclosed device comprises a semiconductor body a portion of which is of one conductivity type, a metal-based layer at a surface of the body and forming an active barrier with said body portion at a plurality of separate areas, and closely-spaced field-relief regions of opposite conductivity type which form with the body portion a p-n junction protruding to a depth in the body below the active Schottky barrier. The separate areas of the active Schottky barrier are located between the closely-spaced field-relief regions. The device has an improved voltage blocking characteristic due to the field-relief regions being contacted at said surface by the metal-based layer and being sufficiently closely spaced that depletion layers extending in the body portion from neighboring field-relief regions merge together under reverse bias of the active Schottky barrier.
In the Schottky diodes made by B. M. Wilamowski, the field-relief regions are in the form of a grid of highly-doped regions of the opposite conductivity type which were diffused from the surface to have a sheet resistance of approximately 100 ohms per square. This sheet resistance is equivalent to a doping concentration of at least 5.times.10.sup.18 cm.sup.-3 at the body surface. The metal-based layer, which was of aluminum in the particular diodes made by Wilamowski, forms an ohmic contact with these highly-doped field-relief regions.
These field-relief regions act as a screen to lower the electrical field near the Schottky barrier at the surface during reverse bias of the Schottky barrier. As a result, the blocking capability is improved by a reduction of the reverse leakage current, a sharpening of the breakdown characteristic and an increase of the breakdown voltage, e.g. from 43 volts to 135 volts. When the Schottky barrier is forward biased the field-relief regions are considered by Wilamowski to be inactive since the forward voltage drop on the Schottky barrier is smaller than the forward voltage drop would be on the p-n junction for the same current density.
It is necessary to enlarge the device area in order to accommodate both the field-relief regions and a given area for the Schottky barrier. This increases the junction parasitic capacitance, and so can reduce the switching speed of the device particularly at low voltages. However, experiments conducted by the present applicants on such Schottky diodes having highly-doped field-relief regions indicate that a significant reduction in the switching speed of this device can also result from minority carrier storage effects as a result of minority carriers injected into the body portion at the p-n junction when the device is forward biased.